In gas heater systems such as a hot water tank heating system, there is normally provided a flue outlet pipe extending into an overhead draft hood with a venting pipe passing up through the ceiling or roof of a room containing the system. Normally, a damper is provided for the overhead venting pipe to prevent heated air in a room from escaping through the vent pipe when the vent pipe is not being used to exhaust gases passing from the flue pipe. Further, the draft hood places the vent pipe in communication with the atmosphere so that when the damper is opened to receive gases from the flue pipe, unexpected downdrafts through the vent pipe in a reverse direction during actual operation of the main heaters can escape to the atmosphere rather than be funneled completely down the flue pipe where it can spread the gas flames and possibly cause a fire.
In addition to the normally provided damper in the vent pipe, it has been proposed to provide a damper in the flue pipe. By closing such a flue pipe damper when the heater is not in operation, heat is retained in the heater system which feature is important in the case of a hot water tank heater for optimum operating efficiency. Such second dampers in the flue pipe are normally not provided, but constitute custom type installations but have proved to be advantageous. In fact, it has been proposed in prior art to provide an appropriate coupling between the vent pipe damper and the flue pipe damper when added so that both could be operated simultaneously. Actually, however, it would be more desirable if the necessity for two separate dampers, even though operated simultaneously, could be avoided and a single damper suffice for all functions.